This invention relates to neutron well logging and more particularly to methods and apparatus for determining, independent of porosity, the nature of hydrogenous fluids present in a formation from characteristics of the epithermal neutron population resulting from irradiation of formations surrounding a well bore with high energy neutrons.
In hydrogenous media, the slowing down time of a high energy neutron to epithermal energy decreases with hydrogen density and is primarily affected by the hydrogen content of the media because of the higher cross section for neutron scattering from hydrogen at epithermal energies and because of the greater average energy loss in elastic scattering from hydrogen. Thus, neutron slowing down time to energies greater than about 0.5 eV is sensitive primarily to formation porosity since the fluid filling the pore spaces, i.e., water and/or hydrocarbons, is hydrogen rich.
As neutron energies decrease below about 0.5 eV, however, further energy loss depends on the type of molecule in which the hydrogen atom is bound. At energy levels between about 0.01 eV and 0.5 eV, the type of molecule in which the hydrogen is bound is the dominant factor in energy loss. Therefore, the slowing down time (die away) of neutrons at energies above about 0.5 eV is primarily sensitive to formation porosity, while at energies between about 0.01 and 0.5 eV, the slowing down time is sensitive to both porosity and the structure of the molecule in which the hydrogen is bound.
The effects of the interaction between high energy neutrons and molecules containing hydrogen atoms in earth formations have been utilized in various ways to obtain valuable information concerning the existence of hydrocarbon deposits in the formations. For example, because hydrogen strongly affects the slowing down of neutrons, and because pores in rock formations are nearly always filled with hydrogenous fluids, detection of the die-away or decay rate of the epithermal neutron population following a burst of high energy neutrons gives a measure of formation porosity.
In addition to porosity determination, it is also important to derive information identifying the nature of the fluid filling the formation pores, i.e., hydrocarbons, water, and the relative quantities of each. Mills, Jr. U.S. Pat. No. 3,497,692 describes a system for determining whether the formation fluid is predominantly oil or water. The differences in count rates of epithermal neutrons having energies above different threshold levels are measured at times following the neutron burst corresponding to the locations of the peaks for 100% water and 100% oil in the formations, respectively. These times depend on the formation porosity and are obtained from calibration curves derived from measurements in formations of known porosity. The differences between the count rates at the respective peak times are then subtracted to indicate the water saturation (Sw) qualitatively. This procedure does not yield a quantitative value for Sw, but a formation fluid which is predominantly water will produce a greater difference at the location of the water peak and vica versa.
A variation on this technique is disclosed in the Mills, Jr., U.S. Pat. No. 4,283,624. The method and apparatus described in this patent employs epithermal neutron die-away (or decay) purportedly to distinguish between free hydrogen, such as is present in oil or water, and hydrogen which is chemically bound to the formation. According to the patent disclosure, epithermal neutron decay rates are measured with two detectors, one having an energy range whose lower limit is below the chemical binding energy of bound hydrogen in the formation, preferably in the range 0.1-1 eV, and the other having an energy range with a lower limit greater than the lower limit of the first detector, preferably in the range 0.5-10 eV. The decay constant measured by the first detector is stated to be indicative of hydrogen in the formation fluids only, while the decay constant measured by the second detector is stated to be indicative of the total hydrogen in the formation, i.e., both in the fluid and bound in the formation. The difference in the decay constants is described as indicating the bound hydrogen porosity.
The patent to Allen et al., No. 4,302,669, discloses a method of identifying fluid content of a formation in which count rates are taken of epithermal neutron populations above two different threshold energies (or with different energy responses) in the energy region below the chemical binding energy of hydrogen in hydrogenous fluid. In addition, count rates are taken during time windows in the thermal equilibrium diffusion period, which count rates are used to correct the count rates determined during the thermalization period to obtain the time constant of the thermalization period. This time constant is used with an independent measurement of porosity to determine saturation Sw.
These known techniques, involving epithermal neutron population measurements above different energy threshold levels, rely on differences between the measurements to provide the results desired, and, where saturation is to be determined, the measured differences are correlated with porosity.